High-pressure transformation behaviour of the cummingtonite-grunerite solid solution

The displacive C2/m <----> P2(1)/m phase transition that occurs with increa sing pressure in cummingtonites has been investigated by means of X-ray sin gle-crystal diffraction at different compositions and degrees of Mg/Fe2+ or der. Unit-cell parameters and a sub-set of intensity data of some "b" and " a" reflections were collected for four natural and two annealed samples as a function of pressure. The phase transition is close to second order in ch aracter, either with a significant contribution from the sixth order term o f the Landau potential or with the fourth order coefficient depending on pr essure via the bare elastic constants. The transition pressure, P-e, obtain ed at the different compositions and degrees of order, displays a plateau a t the grunerite end. The evolution of the spontaneous strain associated wit h the phase transition does not depend on the Mg/Fe2+ substitution or order ing, but it is very sensitive to the impurity (Al, Na, Ca) content.